1. Field of the Invention
The present invention is related to an image projection apparatus which projects an image formed on a display device onto a screen, and the present invention is specifically related to an image projection apparatus that is provided with a display device based on a digital micromirror device which displays an image by turning mirrors that function as pixels.
2. Description of Related Art
A digital micromirror device (hereinafter referred to as a DMD, a product by Texas Instruments Incorporated in USA) has a plurality of micromirrors arranged in a matrix. The tilt angles of the micromirrors are controlled in a binary ON/OFF manner, and by controlling the tilt of the mirrors, it is possible to modulate light incident on the digital micromirror device. Since the DMD has the configuration in which a plurality of mirrors whose tilt angles are variable are densely arranged two-dimensionally, the surface of the DMD includes unevenness resulting from the surfaces of the individual mirrors, and this unevenness functions as a blazed diffraction grating.
A related technology has been proposed to reduce effects of diffraction in the DMD. The related technology proposes a laser processing apparatus that irradiates the mirrors of a DMD with light from a laser light source and diffraction light which is reflected from the mirrors of the DMD is focused on a target to be processed via a condenser lens and an objective lens. In this laser processing apparatus, the DMD is tilted at a predetermined angle so that a light beam that has been reflected on the mirrors is not split into two by diffraction, to thereby enable the light beam that has been reflected on the mirrors to be efficiently transmitted to the target to be processed (see JP-A-2010-44272).
In the laser processing apparatus of the related technology mentioned above, degradation of light transmission efficiency due to diffraction is avoided by tilting the DMD by a predetermined angle (in an example, by 5°) with respect to the optical axis of the objective lens. However, the configuration of this apparatus is not applicable to an image projection apparatus such as a projector. It is true that DMDs are widely used as display devices in image projection apparatuses, but if a DMD is tilted as much as by 5° with respect to the optical axis of a projection lens, blurring occurs in, for example, the periphery of a projected image.
In an image projection apparatus, illumination light is reflected in a state in which the mirror-pixels of the DMD are tilted by a predetermined angle, and thereby, ON-light is outputted as image light in a direction perpendicular to an image display surface of the DMD. On the other hand, illumination light is reflected in a state in which the mirrors are tilted by an angle different from the angle by which they are tilted for the ON-light, and thereby, light that is not necessary for projection is outputted as OFF-light. The ON-light is guided to a screen via a projection lens, but the OFF-light does not enter the projection lens and does not reach the screen. By controlling the tilt of the mirrors in the binary ON/OFF manner in this way, it is possible to display an image on the DMD and project the image onto a screen. However, as described above, due to characteristics of the DMD as a diffraction grating, the OFF-light reflected by the mirrors is outputted by being dispersed according to the diffraction orders, and part of the dispersed light enters the projection lens, which may disadvantageously degrade the contrast of an image that is projected on the screen. The ON-light is also outputted by being dispersed according to the diffraction orders, and part of the dispersed light does not enter the projection lens, which may disadvantageously degrade the brightness of the image that is projected on the screen.
To cope with this, there has been proposed a related technology for preventing the degradation of the brightness of an image projected on the screen. According to the related technology, a pupil of the projection lens is formed to be oval-shaped and the projection lens is arranged such that the OFF-light is dispersed in a direction that coincides with a minor axis direction of the oval-shaped pupil. In this way, the dispersed OFF-light is blocked by the oval-shaped pupil, and thus does not reach the screen, which helps reduce degradation of the contrast of an image projected on the screen (SID 02 DIGEST 46.1: Advances in Contrast Enhancement for DLP Projection Displays, SID Symposium Digest of Technical Papers, May 2002, Volume 33, Issue 1, pp. 1246-1249).
In the image projection apparatus, the light (ON-light, imaged light) reflected by the mirrors of the DMD is also outputted by being dispersed according to the diffraction orders. However, the related technology described above is no remedy for dealing with the dispersed ON-light; on the contrary, the path of the ON-light to the screen is obstructed, and thus the brightness of an image projected onto the screen is degraded. In other words, the related technology is not so configured as to be able to reduce the effect of diffraction on the image light, and as a result, the transmission efficiency of the image light is disadvantageously degraded. In particular, in a case in which the light source is a laser light source, since the wavelength range of laser light is narrow, the effect of diffraction is so great that the transmission efficiency of the image light projected on the screen may be greatly degraded.